The three dimensional structure of the heat stable protease thermolysin has been determined from a 2.3 angstrom units resolution electron density map, and we propose to continue studying the relation between the structure, function and stability of this molecule by X-ray crystallography. We propose to improve the precision of the structure determination by a sequence of refinement steps, and by extending the resolution of the X-ray diffraction data to at least 2.0 angstom units resolution. This project is well under way. As part of the refinement process we will continue to apply Fast Fourier Transform techniques to protein crystallography. These methods can decimate the computing time necessary for refinement of large structures, and also allow new methods of protein refinement which we propose to test with the thermolysin structure. We will continue X-ray structure analyses of biologically interesting proteins. In particular we will continue to study the structure of T4 phage lysozyme which we have recently determined from a 2.5 angstrom units resolution electron density map. Also we will continue the structural study of a bacteriochlorophyll protein, of which suitable crystals have been obtained, and for which we have found two isomorphous heavy atom derivatives which we expect will allow the determination of the three-dimensional structure. DTX* 1GM-20113-2*